A drip irrigation system was used to irrigate single tomato plants, each grown in a thin layer of an inert root medium supported between two glass plates. Sodium chloride was added to the medium and, with irrigation, was distributed unevenly through the root medium. The lowest concentration occurred immediately below the dripper outlet, the highest at the edge of the wetting pattern. Different amounts of sodium chloride were added initially, and these resulted in different concentrations of salt at the edges of the wetting patterns.
With increasing amounts of salt added to the system, root growth was restricted to smaller volumes of root-zone space immediately below the dripper outlet, where minimum salt concentrations occurred. The percentage root distribution was higher immediately below the dripper outlet with increasing salt loading of the root zone, but the weight of both roots and plant tops decreased with increasing salt.
We infer from the results that an essential factor in the management of drip irrigation under saline conditions is to provide a large enough volume of wet soil with low salt concentration to minimize contact between roots and zones of high salinity. This should prevent growth depression caused by uptake of Na+ or Cl- to toxic concentrations, osmotic effects, or restriction of the size of the root system.